Conventionally, as means for inputting an image in a personal computer, a facsimile machine, etc., adopted is an image input device (an image scanner, for example) adopting a combination of a 1D image sensor and a mechanical scanner. For such image input device, a mechanical scanning is necessary to scan an image in a document either by moving the 1D image sensor relative to the document or moving the document relative to the 1D image sensor. A problem therefore arises in that the speed of reading images is lowered, or the apparatus cannot be reduced in size and weight as desired.
On the other hand, various types of close coupling image sensors which do not require mechanical scanning have been proposed. Such two-dimensional image sensors generally include pixels arranged in a 2D matrix form, each pixel being made up of a pixel selection transistor which serves to prevent crosstalk, and a photodiode or a photo transistor, which functions as a photosensor.
The foregoing two-dimensional image sensor is typically arranged such that the amount of charge generated according to an amount of light incident on each photosensor is stored in each pixel, and an amount of charge is read from each pixel by conducting pixel selection transistors in order by electric scanning. For such two-dimensional image sensor, for example, ITE Technical Report Vol. 17. No. 16. pp 25 to 30 discloses a test sample adopting a pin photodiode as a photosensor.
Incidentally, to realize a high density pixel, the structure which adopts a pixel selection transistor and a photosensor as one integral part has been proposed. For example, Japanese Patent No. 2796336 (Japanese Laid-Open Patent Application No. 2-246272/1990 Tokukaihei 2-246272/2000 (published on Oct. 2, 1990), and Japanese Patent No. 3019632 (Japanese Laid-Open Patent Application No. 6-132560/1994 Tokukaihei 6-132560/1994 (published on May 13, 1994)) discloses the structure wherein the photosensor itself functions not only as a photosensor but also to select pixel to realize a pixel with a smaller area.
For the described beneficial features which realize high speed image reading, lighter weight, thinner structure without requiring mechanical scanning, various types of close coupling two-dimensional image sensors have been proposed, but none of them have not yet been used in practical applications.
Essential factors which hinder the practical applications of the foregoing close coupling image sensor include the complex structure of the pixel and the array. Specifically, to fabricate a photosensor, a larger number of processes are needed, or the pixel of more complicated structure is needed, or a larger pixel area is required as compared to the case of fabricating the TFT array adopted in the active matrix liquid crystal array subjected to mass production, and a higher resolution is therefore difficult to be realized.
For the arrangement wherein the photosensor itself serves not only as a photosensor but also as a member for selecting the pixel, such complicated structure that an inverse stagger type thin film transistor and a stagger type thin film transistor are combined by adopting a semiconductor layer of a monolayer structure is adopted, and the complicated manufacturing processes are therefore needed.
The present invention is achieved in view of the foregoing problems associated with the conventional structure, and it is therefore an object of the present invention to provide a photoelectric converter which realizes a higher resolution by simplifying the pixel structure to be adopted in a photosensor, and which permits the manufacturing process to be simplified, and also to provide an image input device and a two-dimensional image sensor adopting such photoelectric converter.